The present invention relates generally to input devices for interfacing with computer systems, and more particularly to computer input devices that provide spatial information about a three-dimensional object to computer systems which provide a representation of the object.
Three-dimensional (3-D) digitization is the process of sensing a three-dimensional object and creating a three-dimensional representation of the object which can be manipulated as digital data by a computer system. Detailed and accurate three-dimensional models can be created and manipulated by computer systems for use by animators, engineers, scientists, designers, architects, and others who have a need for a realistic three-dimensional, manipulable model derived from a real, physical object.
Many types of devices are available which can digitize three dimensional objects. A common type of digitizing apparatus utilizes a probe device, such as a stylus or other pointer, to trace over surfaces a three-dimensional object and thereby provide the spatial coordinate data of the object to a host computer system. The host computer system can sample the probe device to receive discrete data points at different spatial coordinates. The points can be joined together and displayed as a xe2x80x9cmesh representationxe2x80x9d, which is a wire-frame type model comprising a set of vertices with interconnecting lines or polygons (typically called a xe2x80x9cdata setxe2x80x9d). A realistic 3-dimensional shaded model can be created by a computer system from the mesh representation of an object.
A common type of probe apparatus utilizes mechanical linkages and sensors to determine the position of the stylus or other probe that is tracing the three-dimensional object. The stylus is fixed to one end of a series of mechanical linkages, and the other end of the linkage chain is connected to a base fixed to a stationary surface. Sensors can be included in joints of the linkage chain to sense the relative orientation of linkages, and therefore the stylus, are located with respect to the base. The angle data read by the sensors can be converted into coordinate data by a microprocessor interface or by the host computer system.
Problems with prior art 3-D digitizing methods often occur because the user is too constrained when tracing an object that is to be digitized. Typically, the user is required to trace the object along particular surfaces of the object and in particular directions on the surfaces. This can often cause errors in the resulting mesh representation when a surface is traced in the wrong direction or points are not connected properly. In addition, a user often cannot view a resulting mesh representation until the entire object or a large portion of the object has been traced by the digitizing apparatus. This allows further errors to be introduced into the mesh representation and causes more time to be spent correcting mesh representations, since the user cannot immediately determine if a point has been entered inaccurately.
Other inaccuracies are introduced when using prior art methods and apparatuses for digitizing 3-D objects. For example, a user may wish to move or rotate an object that has been only partially digitized to gain access to hard-to-reach surfaces on the object. The object can be placed on a rotary table, for example, to assist in easily rotating the object. However, once the object is moved, the host computer system can no longer develop a mesh representation from the old position of the object. In the prior art digitizers, a user must first select three or more points on the object, move the object to the desired new position, and re-select the same three or more points on the object at the new position. The host computer can transform the coordinates, taking into account the new position of the object, and thus continue developing the mesh representation. However, such a procedure typically introduces error into the mesh representation, since it is difficult to accurately re-select the same points at the object""s new position. In addition, it is time consuming to perform such a procedure and unduly interrupts the digitizing process.
In many prior art digitizing apparatuses, the user is further constrained by the joints of the linkage assembly. Since wires are routed through the joints to carry electrical signals from sensors located therein, the joints typically include stops which limit the motion of a joint to under 360 degrees to prevent twisting and stressing the wires. However, this limited movement can inconvenience the user when tracing an object, especially when a limit to a joint is reached in a particular direction and further movement of the stylus in that direction is required to trace the surface of the object.
Further problems exist with the prior art digitizing apparatuses. Since the digitizing apparatuses must accurately sense an object to provide valid coordinate data to the computer system, calibration of the sensors of the apparatuses is of paramount importance to compensate for variations in the mechanical structure of the linkages and joints. In the prior art, calibration is typically accomplished by placing the stylus or other probe device at known locations in space and to record position readings at those known locations. Deviations between the known locations and the measured position readings can be used as error parameters to fine tune calibration parameters. However, such a calibration method requires that known locations be defined and that the stylus be accurately fixed to those known locations. This can require expensive precision fixtures. In addition, this calibration method is a slow and careful one, and can be tedious.
In addition, other initialization procedures of the prior art can be awkward or time consuming. Digitizing apparatuses often use less expensive relative sensors which detect a change in the position of a linkage of the digitizing apparatus rather than reading an absolute angle for the position of the linkage. When using such relative sensors, a xe2x80x9czeroingxe2x80x9d procedure is often accomplished each time the apparatus is powered up to provide reference starting angles for the relative sensors. For example, in the prior art, zeroing can be accomplished by moving each individual joint to a stop of the joint and starting angles are xe2x80x9czeroedxe2x80x9d at those points. However, in digitizing apparatuses having four, five or six degrees of freedom, this procedure can be very time consuming to move each individual joint each time the apparatus is powered up. Other digitizing apparatuses use a xe2x80x9chome positionxe2x80x9d to provide starting angles. The stylus is placed into a receptacle on the base of the apparatus such that the reference starting angles for all the sensors is known when the apparatus is powered up. However, having the receptacle for a home position on the base of the apparatus typically requires a larger base that covers a larger surface area on a support surface such as a tabletop, which can be inconvenient. In addition, the more degrees of freedom on a digitizing apparatus, the more joints that need to be zeroed between the base and the probe. The greater the number joints to be zeroed, the greater the chance for error to be introduced in the zeroing process.
The present invention provides a measuring system for measuring three-dimensional (3-D) coordinates. In the preferred embodiment, the probe apparatus is used to digitize three-dimensional objects into a mesh representation manipulable by a computer system. Various improvements to a probe arm linkage, a calibration method and zeroing method for a probe apparatus, a rotary table for supporting an object to be digitized, a method for developing a mesh representation by a computer system, and a method for assembling a probe arm linkage are described herein.
A probe apparatus of the present invention for sensing the position and orientation of a probe, such as a stylus, includes a first joint member coupled to the probe that provides two degrees of freedom to the probe, and a first linkage rotatably coupled to the first joint member. Further, a second joint member is rigidly coupled to the first linkage and provides one degree of freedom to the probe. A second linkage is rigidly coupled to the second joint member, and a third joint member is rotatably coupled to the other end of the second linkage. The third joint member provides two degrees of freedom to the probe. Finally, a support base is coupled to the third joint member for supporting the probe apparatus. Transducers of the probe apparatus provide angular signals for the provided degrees of freedom to describe a position and orientation of the probe. Preferably, the probe apparatus provides 3-D data describing a 3-D object to the computer system. An electronics interface is included within a housing of said support base and provides the angular signals from the sensors to a computer system. The joint members can include a multistage stop joint of the present invention which provides over 360 degrees of rotational movement about an axis. To allow the first joint member to be small and the stylus to be easily manipulable by the user, the transducer for one of the first joint member degrees of freedom is positioned in the second joint member. The first joint member can provide three degrees of freedom to the probe in an alternate embodiment.
A method of the present invention for calibrating a probe apparatus for measuring 3-D coordinates, such as the probe apparatus described above, includes a step of sampling multiple orientations of the stylus as the orientation of the stylus is varied at an arbitrary point within the probe""s work volume. The position of the stylus tip remains fixed while the user varies the orientation of the stylus. Multiple orientations are sampled from data provided by sensors on the probe apparatus. Spatial coordinates are then determined for the stylus at each of the sampled orientations of the stylus. Next, error values between the spatial coordinates of the sampled orientations are determined. Finally, the probe apparatus is optimized by determining calibration parameters based on the error values and using the calibration parameters when determining the position and orientation of the probe during normal operation. Preferably, previous calibration parameters are loaded from a memory device before the calibration process and are adjusted to become new calibration parameters. The adjusted parameters are preferably stored on an EPROM memory device which is unable to store data over previously written data stored in the EPROM. The adjusted calibration parameters are stored as a most recent set of calibration parameters in a specific section of the EPROM such that, when calibration parameters are retrieved during operation of the probe apparatus, only the most recent set of calibration parameters are retrieved. The calibration steps can be repeated when the stylus is placed at a different position in the selected volume and the sampled orientations from both of the positions can be used when determining the error values.
A method of the present invention for zeroing the sensors of a probe apparatus of at three-dimensional coordinate measuring system having relative sensors, such as the probe apparatus described above, includes placing the stylus (or other probe) in a receptacle positioned on one of the joints or one of the linkages of the probe apparatus. This receptacle preferably takes the form of a small bore or shallow divot. The linkage arm assembly can be in only one possible configuration while the stylus is positioned in the receptacle; this one configuration is known as a xe2x80x9chome position.xe2x80x9d An indication is then received to zero the sensors of the probe apparatus, such as powering up the probe apparatus. Starting angles are then assigned to the sensors when the probe apparatus is in the home position. The starting angles provide a zero angle reference for the sensors of the probe apparatus. Preferably, the starting angles have previously been calibrated for the particular probe apparatus that is undergoing the zeroing process.
A rotary table of the present invention is for use with a 3-D digitizing system that includes a probe apparatus for measuring 3-D coordinates on an object resting on the rotary table, such as the probe apparatus described above. The rotary table includes a support base and a turntable which rotates. A sensor is coupled to the support base which measures an angular rotation of the turntable and provides the angular rotation to a host computer system. The host computer system includes the turntable rotation in a determination of the position and orientation of the probe when the probe is contacting the object resting on the turntable. The angular rotation is included in the probe determination when the probe has contacted the object before the turntable has been rotated and after the turntable has been rotated so that the object can be referenced at its new position. The sensor is preferably positioned near the center of said turntable such that a shaft of the sensor is coupled to the turntable. The turntable also preferably includes a receptacle positioned near a periphery (or the center) of the turntable for receiving the probe in an initialization procedure for locating the turntable relative to the probe apparatus. For example, the initialization procedure can include placing the probe in the receptacle, rotating the turntable while the probe is positioned in the receptacle, sampling multiple positions and orientations of the probe as the turntable is rotated, and determining the position and orientation of the rotary table relative to the probe apparatus using the sampled positions and orientations of the probe. Alternatively, the support base of the table is coupled to the base of the probe apparatus such that the position and orientation of the rotary table is fixed relative to the probe apparatus.
A method of the present invention for developing a mesh representation of a three-dimensional object by a computer system includes receiving a data point from a probe corresponding to a surface point on a surface of a three-dimensional (3-D) object. The data point is added to an end of a current contour line of the mesh representation, where the current contour line includes data points corresponding to surface points on the surface of the 3-D object. A triangle is then created in the mesh representation that includes a data point of the current contour line, a data point of a previous contour line, and a third data point from either the current contour line or the previous contour line. The previous contour line is adjacent to the current contour line and includes data points previously received from the probe. The triangle is created only when the current contour line is not the first and only contour line of the mesh representation. Preferably, data points of the triangle are chosen based on the distance between data points of the current contour line and data points of the previous contour line. In addition, a normal vector is assigned to the created triangle that indicates the exterior surface of the mesh representation. The orientation of the normal vector is based on orientation data included in the data points of the triangle. The triangle is displayed on a display screen of the host computer immediately after the triangle is created, thus displaying the mesh representation incrementally. When the received data point is the last data point of the current contour line, a triangle is created for each data point of the previous contour line that is not included in a triangle in a xe2x80x9cclose meshxe2x80x9d process. The data points are provided to the host computer from the probe as a user is tracing the probe across the surface of the object. The data points can be provided when a user control is activated or after the user traces the probe a minimum distance on the object.
A method of the present invention for providing a selection template that allows commands to be selected by a probe apparatus includes defining a template area as the selection template within a selected volume that the stylus of the probe apparatus can reach. A selection area is defined within the template area and is associated with a command to the host computer. The command associated with the selection area is provided to the host computer to select a function of said host computer or of the probe apparatus when the tip of the stylus is positioned within the selection area. Preferably, a plurality of selection areas are defined within the template area that are each associated with a different command. The selection areas can include indicia such as icons and labels. Some of the commands can manipulate the mesh representation of the object displayed by the host computer.
A method of the present invention is also described for assembling a linkage assembly including a plurality of joints and a linkage used in a probe apparatus for measuring three-dimensional coordinates, such as the probe described above. Two joint fixtures are positioned a desired distance apart and have a desired angle offset from each other. A joint of the linkage assembly is placed in each of the joint fixtures. The joints can be moved relative to the linkage connecting the joints so that the joints fit in the joint fixtures. The joints are then bonded to the linkage while the joints are placed in the joint fixtures. The linkage is preferably made of graphite, which is well adapted to the bonding process.
The features of the present invention allow for a more accurate, more convenient, and less expensive coordinate measuring and 3-D digitizing system than the systems of the prior art. Specifically, the probe arm apparatus provides a lightweight, accurate device for the user to handle. The calibration method allows a probe to be positioned at an arbitrary point in a volume and thus avoid expensive precision fixtures. The zeroing method is more accurate and allows only one possible physical configuration of the probe arm to be in the home position. The method of the present invention for assembling linkages and joints for a probe apparatus allows accurate lengths and angles between joints. The rotary table allows an object to be re-oriented by a user during digitization without a cumbersome procedure and with great accuracy. The method for developing a mesh representation allows a user to incrementally view a mesh representation and quickly and accurately finds the orientation of the mesh polygons. Finally, the selection template allows the user to conveniently select commands and functions of the host computer and probe during a digitization process.